Symbols for several arithmetical (`+', `-', ...) and relational (`<',
`>=', ...) operators are overloaded so that they may denote the operators
for several different number systems, particularly num (natural numbers),
int (integers) and real (real numbers). The choice is normally made based
on some known types, or the presence of operators that are not overloaded for
the number systems. (For example, numerals like 42 are always assumed to be
of type num, while the division operator `/' is only defined for real.)
In the absence of any such indication, a default choice will be made. The
effect of prioritize_num() is to make num, the natural number type, the
default.